determining the dispersive ratio of glass, &c. 249 
13. Table showing the aberration of a lens for parallel rays to 
the four different indices *1500, "1510, *1520, *1530. 
p ~ the 
value of 
aberra- 
Ratio of surfaces, or values of q for different indices. 
I.500 
1-510 
5-20 
5 - 3 o 
tion. 
1 
? 
? 
i 
1-05 
Imaginary. 
Imaginary. 
Imaginary. 
-180 
i -io 
•292 
•291 
•296 
• 3°3 
, ’ I 5 
•380 
•374 
•377 
•380 
I -20 
•445 
•445 
.446 
•447 
I- 2 5 
*5 1 5 
•510 
•506 
-509 
1-30 
•572 
•570 
•568 
-567 
>•35 
• 6 3 S 
•627 
•625 
•626 
1-40 
•689 
•686 
•683 
•683 
1-45 
750 
•743 
• 74 ° 
•740 
1 -50 
■803 
•800 
-798 
-798 
>’55 
•865 
•858 
-855 
•855 
i-6o 
•921 
• 9>7 
* 9*3 
' 9*3 
1-65 
•9/9 
•97 5 
•973 
.972 
170 
1*042 
>-°37 
>•034 
1-034 
>•75 
1-103 
1-098 
1-096 
1-095 
1 -8o 
i- 166 
1-161 
i *>59 
1**59 
>•85 
1-230 
1-226 
1-223 
1-223 
1 -90 
1-296 
1-292 
1-289 
1-290 
1-95 
1-360 
1-356 
>•357 
>'359 
2-00 
>•434 
1 * 43 1 
1-429 
1-430 
2-05 
1-506 
1-503 
1-503 
>•504 
2 ’ IO 
1-581 
1-578 
1-579 
1-580 
2-15 
1-658 
1-657 
1-658 
1-659 
2-20 
1-738 
1738 
1-738 
>' 74 > 
Example. 
14= Let it be proposed to compute the curves for an achro- 
matic object-glass ; the data being as below, viz. 
Index of plate = 1-515, of flint = r 6 oo, 
Dispersive ratio = • 66 , diameter 5| inches, focus 80 inches,, 
Here a = ‘515, a' ~ -6oo, d = • 66 , f" = 80. 
First f" ( 1 — d) = 80 x -34 = 27-20 = f 
»d /" ^7^- = 4i°2i = /'. 
K k 
MDCCCXXVII. 
